In many types of printers, a printhead (for example, an inkjet printhead), including an array of marking elements, is controlled to make marks of particular sizes, colors, etc. in particular locations on recording media (sometimes generically called paper herein and used interchangeably with the term “media”) in order to print a desired image. In some types of printing systems (sometimes termed “page-width printers”) the array of marking elements extends across the width of the recording medium and the image can be printed one line at a time as the recording medium moves relative to the printhead. In other types of printing systems (sometimes termed “carriage printers”) the printhead or printheads are mounted on a carriage that is moved past the recording medium in a carriage scan direction as the marking elements are actuated to make a swath of dots. At the end of the swath, the carriage is stopped, printing is temporarily halted, and the recording medium is advanced. Then another swath is printed, so that the image is formed swath by swath.
In order to produce high quality images, it is helpful to provide information to the printer controller electronics regarding the type of the recording medium, such as whether it is a photo paper or plain paper, for example. For inkjet printing, knowing the type of recording medium before electronically preparing the image for printing is advantageous, because differences in ink-recording medium interactions on different recording medium can result in poor image quality, if the amount and timing of ink deposition is not controlled appropriately for the type of recording medium in the printer.
Using an optical sensor to detect the type of recording medium in a printer is known in the prior art. Some examples are disclosed in U.S. Pat. Nos. 5,764,251; 6,291,829; 6,325,505; 6,386,669; 6,561,643; 6,838,687; 6,914,684; 6,984,034; and co-pending U.S. patent application Ser. No. 12/037,970. Such an optical sensor assembly for recording medium type detection can optionally be attached to the printhead carriage of a carriage printer. In the same way that the printhead can mark on all regions of the paper by the back and forth motion of the carriage and by the advancing of the recording medium between passes of the carriage, a carriage-mounted optical sensor is able to provide optical measurements, typically of optical reflectance, for one or more regions of the paper. Other types of optical sensors include stationarily-mounted sensors that are positioned near the paper-advance path of the printer, so that one or more regions of the paper can be viewed by the sensor as the paper moves past it.
An optical sensor assembly for recording medium type detection typically includes one or more photosensors and one or more light sources, such as LED's, mounted such that the emitted light is reflected off the surface of the recording medium, and the reflected light is received in the one or more photosensors. LED's and photosensors can be oriented relative to each other such that the photosensor receives specular reflections of light emitted from an LED (i.e., light reflected from the recording medium at the same angle as the incident angle relative to the normal to the nominal plane of the recording medium) or diffuse reflections of light emitted from an LED (i.e., light reflected from the recording medium at a different angle than the angle of incidence).
Typically, the photosensor signals for specular and/or diffuse reflections of light from the surface of the recording medium are amplified and then processed to separate the signal from the background noise. The processed signal characteristics are then compared with known signal characteristics for different recording medium types, and the present recording medium type is identified.
One known way in which recording medium types can be distinguished from one another is the spatial frequency of the variation of optical reflectance from the surface of the recording medium. It is known, for example, that photo papers made for inkjet printing tend to have a specular optical reflectance that has a spatial frequency of variation that is dominated by high frequency components, and that plain papers tend to have a specular optical reflectance that has a spatial frequency of variation that is dominated by comparatively lower frequency components.
Automatic detection of recording medium type in a printer is desired to be: a) highly accurate, so that the correct recording medium type is dependably identified; b) fast, so that printing throughput is not adversely impacted due to waiting for identification of the recording medium type; c) robust, so that aging or contamination of the light source or photosensor, or different environmental conditions do not degrade the reliability of recording medium type identification; and d) simple so that it can be done with low cost.
Different trade-offs in recording medium type identification requirements can be made in different types of printing systems. For example, U.S. Pat. Nos. 6,325,505 and 6,561,643, disclose performing a Fourier transform on the reflectance data to quantify the spatial frequency components. This may be appropriate for printers that are always connected to a host computer, because Fourier transform analysis can require extensive processing in order to be done quickly. However, all-in-one printing systems that include a scanner as well as a printer; are sometimes operated in a standalone mode for copying photos and documents, and it is important that the recording medium type identification still be fast, accurate, reliable, and simple. In low-cost all-in-one printing systems the system controller may be a “system-on-a-chip” controller, which is inexpensive, but may not have the processing bandwidth for complex calculations for recording medium type identification.
Some types of simple and fast signal processing methods for photosensor reflectance data for recording medium type identification are found to be highly accurate when the printing system is new, but as the sensor assembly ages or the optical components become coated with ink mist or particulates, or encounter extreme environmental conditions, the resulting shift in the processed signal characteristics can cause occasional misidentification with a set of known signal characteristics so that accuracy of recording medium type identification is not as high as it was initially. As a result, occasional recalibration may be required in order to restore the accuracy. While such recalibration can be automatic and programmed into printer firmware for implementation, it is preferable in some embodiments to provide a signal processing method that is more robust in its accuracy of recording medium type identification.
Therefore, what is needed is a new method of signal processing that is simple, fast, robust, and effective in analyzing a frequency distribution for accurate identification of a type of recording medium or other body types by reflections from their surfaces.